(i) Field of the Invention
This invention relates to a method and apparatus for the production of expanded metal mesh sheet and, more particularly, relates to a one-step method and apparatus for the production of expanded metal mesh sheet for use in lead-acid battery manufacture.
(ii) Description of the Related Art
The prior art discloses rotary methods for expanding lead strip for use in the manufacture of battery plates. Such methods employ clusters of tools arranged sequentially for preforming and slitting the strip in a first step and completion of slitting of the strip in a second step. Sequential methods have the inherent problems of synchronization of steps, such as roll-to-roll synchronization, requiring certain registering and tracking considerations.
Sequential methods use different tooling for the different steps with the result that lead strip is not “symmetrically processed”, in that opposite sides of the strip are not always subjected uniformly and simultaneously to the same pressures, forces, stretching, and the like. In one predominant method in the prior art, a three-shaft cluster of tooling is arranged sequentially with three different tooling devices, namely a “preformer”, a “preform slitter” and a “slitter”, such that a two-step method results. The preformer and preform slitter form the metal strip by stretching and cutting in a first step and the slitter completes the slitting in a second step.
Wires and nodes on opposite sides of the expanded strip produced by the stretching and forming according to the prior art are not uniform and are not symmetrical. The profile and shape on one side is not the mirror image of the other side resulting in a number of imperfections and defects. This becomes even more significant when higher elongation targets are desired in order to produce lighter grid electrodes for batteries.
Cominco U.S. Pat. No. 4,291,443 issued Sep. 29, 1981 and U.S. Pat. No. 4,315,356 issued Feb. 16, 1982, both included herein by reference, disclose the geometric relationship of conventional 3-shaft cluster tooling or spaced-apart roll pairs employing two sequential steps, i.e. preforming, wherein the lead strip is slit and stretched to form wires that are still solidly connected and not in a form to be pulled apart, and slitting, wherein alternate slits in the nodes are made to allow subsequent expansion to complete the process.
Cominco U.S. Pat. No. 4,297,866 issued Nov. 3, 1981, also incorporated herein by reference, discloses a sequential two-step process for the production of symmetrical slit wires deformed out of the plane of the strip having a trailing portion of the wire longer than the leading portion for improved stretchability of the wires.
Forming of the strip in a one-step process has been discounted and not achieved to date because of perceived intricacies of the grid design and physical limitations of the grid components, particularly fore-shortening and rippling of the strip. U.S. Pat. No. 1,472,769 issued Oct. 3, 1923 discloses a method and apparatus for expanding metal sheet between opposed rollers in which wire strands and bands are slit in the sheet, slit strands are returned to the plane of the sheet by flattening rolls, longitudinal corrugations are then formed in alternate series of bands in reverse directions to stretch the strands, and the sheet then laterally expanded to form a mesh. It was believed necessary to incorporate the flattening and longitudinal corrugating steps in the process for the formation of uniform meshes.